Metal surfaces such as reactor walls and internals, heat exchanger tubes, and other piping and equipment used in conversion of hydrocarbon-containing feedstreams often become coated with coke. The type of reactions in which such coking can occur are well known. Such reactions include thermal cracking for the production of olefins, in particular ethylene, reforming and catalytic cracking for the production of motor fuels and cyclization reactions for the production of aromatics are just of few of such processes. In such reactions, coke formed as a by-product usually coats the inside of a substantial portion of the metal surfaces in the environment of the convened hydrocarbon products or reactants.
Coke accumulation has a number of drawbacks. Accumulated coke reduces the diameter of piping and flow areas within the vessel causing reduced reactor throughputs or increased pressure drops. The insulating effects of coke accumulation pose special problems in heat exchanger tubes where additional heat input and higher temperatures are needed to compensate for the presence of the coke. Higher tubewall temperatures cause faster deterioration of many heat exchanged surfaces. In addition, in some processes, large chunks of coke can break off of metal surfaces and jam or block downstream process equipment. Therefore, whether as part of routine maintenance, or an emergency shut-down, coke formation complicates the operation of hydrocarbon conversion processes.
A number of methods are known for reducing coke formation in hydrocarbon conversion processes. One such method is steam dilution of hydrocarbon vapors to reduce the level of homogeneous coking. Sulfur compounds will also act to inhibit coking in many processes. However, both steam and sulfur can deleteriously affect many hydrocarbon conversion processes. Excess water in the form of steam deactivates many catalyst compositions. Similarly, sulfur poisons a variety of catalyst systems that are used in coke forming processes.
Other methods of inhibiting coke formation include chemical treatment of metal surfaces in hydrocarbon conversion processes. It is known from U.S. Pat. No. 4,099,990 that the application of silicon compounds to substrate surfaces can inhibit coking of such surfaces in many hydrocarbon conversion processes. The '990 patent teaches the chemical vapor deposition of an alkoxysilane to produce a silicon film on metal surfaces. By this method a the silica film on metals substrates.
The multiple step procedure makes deposition of the silica film a complicated process. Therefore, use of the deposition procedure can add considerable expense to the manufacture of processing equipment. In addition to cost, the availability of equipment and experienced personnel makes the procedure impractical for many field installation situations. A silica deposition technique, readily adaptable to field application, would facilitate the use of such deposition techniques in many hydrocarbon conversion processes that require field fabrication, field assembly or on site repair of equipment and vessel internals. As a result any method of more easily forming a coke inhibiting layer on a metal surface would be highly useful.